This invention relates to the production of carbon black. More particularly, this invention relates to minimizing the degredation of the carbon black reactor refractory from the action of meltable solids contained in hydrocarbon feedstocks. Even more particularly, this invention relates to the recovery of a hydrocarbon/ash concentrate while reducing deposits of ash on the interior of the carbon black reactor.
Carbon black feedstocks for the "oil furnace process" are most often obtained from a fluid catalytic cracking operation employing a silica alumina or similar catalyst system. By "oil furnace process" it is meant a process for making industrial, tire carcass or tire tread grade carbon blacks. In the fractionation of the effluent from the fluid catalytic cracker, the bottom product, frequently called a slurry, contains a suspended quantity of very finely divided catalyst. Much of the catalyst is removed by settling or clarifying, but these bottoms commonly contain from about 0.04% to about 0.1% finely divided catalyst, or ash, and other solids. These clarified bottoms are normally the source of feedstocks for the production of carbon black by the oil furnace process.
Before these feedstocks can be charged to the carbon black reactor, the ash and other solids content must be reduced to a lower level, i.e., at least below about 0.03%. Higher levels of ash and other solids result in excessive deposition on and attack of the surface of the reactor refractory liner, with eventual deterioration of the refractory liner in the combustion and reaction zones. This deterioration results from both a softening of the surface of the refractory liner, as well as deformation in the refractory dimensions within the combustion and reaction zones. This deformation generally restricts flows, changes flow patterns, and increases pressure drop (energy loss) through the reactor. Hence, this condition is to be avoided.
It is avoided by reducing the ash and other solids content of the feedstock to below about 0.03% by weight. There are several ways to remove these solids, e.g., by centrifuging. From the centrifuge, a sludge containing a hydrocarbon/ash concentrate comprising from about 1% to about 3% of the feedstock is generated. The centrifuged hydrocarbon is charged to the reactor and the sludge may be discarded. However, for the typical sized carbon black plant, discarding the hydrocarbon contained in the sludge may represent an annual loss of several hundred thousand dollars at today's price of feedstocks. It is therefore desired that this material be recovered.